Exercise induction at expression immediate early gene (c-Fos, ARC, EGR-1) in the hippocampus: a systematic review

ABSTRACT. The immediate early gene exhibits activation markers in the nervous system consisting of ARC, EGR-1, and c-Fos and is related to synaptic plasticity, especially in the hippocampus. Immediate early gene expression is affected by physical exercise, which induces direct ARC, EGR-1, and c-Fos expression. Objective: To assess the impact of exercise, we conducted a literature study to determine the expression levels of immediate early genes (ARC, c-Fos, and EGR-1). Methods: The databases accessed for online literature included PubMed-Medline, Scopus, and ScienceDirect. The original English articles were selected using the following keywords in the title: (Exercise OR physical activity) AND (c-Fos) AND (Hippocampus), (Exercise OR physical activity) AND (ARC) AND (Hippocampus), (Exercise OR physical activity) AND (EGR-1 OR zif268) AND (Hippocampus). Results: Physical exercise can affect the expression of EGR-1, c-Fos, and ARC in the hippocampus, an important part of the brain involved in learning and memory. High-intensity physical exercise can increase c-Fos expression, indicating neural activation. Furthermore, the expression of the ARC gene also increases due to physical exercise. ARC is a gene that plays a role in synaptic plasticity and regulation of learning and memory, changes in synaptic structure and increased synaptic connections, while EGR-1 also plays a role in synaptic plasticity, a genetic change that affects learning and memory. Overall, exercise or regular physical exercise can increase the expression of ARC, c-Fos, and EGR-1 in the hippocampus. This reflects the changes in neuroplasticity and synaptic plasticity that occur in response to physical activity. These changes can improve cognitive function, learning, and memory. Conclusion: c-Fos, EGR-1, and ARC expression increases in hippocampal neurons after exercise, enhancing synaptic plasticity and neurogenesis associated with learning and memory.


INTRODUCTION
T he group of genes that are activated by neurons is known as immediate early genes (IEG) 1 .IEG is essential in brain function, particularly in the synaptic process.IEG neurons are regulated by cellular and synaptic developmental responses 2 .The expression of c-Fos, ARC, and early growth response 1 (EGR-1)/zif268 represents subsets of IEG genes.These genes are rapidly and selectively controlled in hippocampal learning and memory 3 .
Numerous studies explored neural IEG 4 .One extensively studied gene is the cAMP-responsive element-binding protein (CREB), which plays a crucial role in regulating synaptic development and plasticity 5 .IEG expression is influenced by nerve stimulation; for instance, EGR-1 messenger RiboNucleic Acid (mRNA) depends on N-methyl-D-aspartate (NMDA) glutamate receptors, while c-Fos is independent of nerve stimulation 6,7 .As part of the IEG family, ARC is an effector involved in neural signaling pathways, not a transcription factor.However, the ARC gene is transcribed in response to neuronal activity and synaptic activation of neuron dendrites 8,9 .
Several IEGs play a role in encoding transcription factors and transiently enhancing transcription in the rat dentate gyrus following afferent stimulation induced by long-term potentiation (LTP) synaptic plasticity or persistent and long-term potential dependent on the activity of associative memory mechanisms.Among these genes, EGR-1/zif268 is most explicitly linked to LTP because it is induced in all LTP-triggering situations and shows a very high correlation with LTP duration 8,10 .
This review aims to elucidate the role of IEG in synaptic plasticity in exercise-induced learning and memory.While many other review articles have discussed IEG in synaptic plasticity, none have specifically delved into the role of exercise-induced IEG genes in synaptic plasticity.

METHODS
We conducted a comprehensive search of online literature databases, including Scopus, PubMed-Medline, and ScienceDirect.The original articles in English were selected using the following keywords in the title: (Exercise OR physical activity) AND (c-Fos) AND (Hippocampus), (Exercise OR physical activity) AND (ARC) AND (Hippocampus), (Exercise OR physical activity) AND (EGR-1 OR zif268) AND (Hippocampus) between 2013 and 2023.Our analysis was limited to experimental studies with exercise interventions, and we included articles in English published between 1987 and 2022 (Figure 1).Studies in animals that involved exercise interventions and included neurocognitive impairment were considered.However, we excluded systematic reviews, literature articles, and simplified and expanded abstracts published in proceedings and book chapters.Studies related to neurodegenerative diseases, such as Huntington's disease and diabetes mellitus, were also excluded.

Outcomes
C-Fos, ARC, and EGR-1/zif268 are induced in neurons during neural activity in the hippocampus, including Morris' water maze [24][25][26][27] .Physical activity upregulates neurotrophins and neuropeptides [28][29][30] in long-term hippocampal potentiation (LTP) 31 .Molteni et al. (2002)  showed in research that physical exercise increases hippocampal gene expression, including IEGs, associated with neuronal plasticity 32 (Figure 2).c-Fos expression during exercise c-Fos is one of the first neuronal transcription genes whose induction is activity-dependent 33 due to cAMP and Ca2+ stimulation by activating the CREB complex 34 .Increased c-Fos expression is an indicator of neural activation, as demonstrated by behavioral training in learning 35 , cognitive performance 36,37 , and memory formation 38 .
Physical exercise induces c-Fos expression in the rat hippocampal central nervous system, contributing to neuroplasticity 35,39 , increased neural activity 40 , and spatial memory coding 41 .c-Fos expression also plays a role in autonomic and somatomotor control and extends to other parts of the brain 42 .Additio ally, physical exercise increases the c-Fos expression through serum insulin-like growth factor-I (IGF-I) as a neuroprotective factor 43 .
Previous studies showed that physical exercise can improve cognitive function by increasing hippocampal neurogenesis 44 .This is supported by evidence from voluntary wheel running exercise, which maintains a stable level of neurogenesis through increased progenitor cell differentiation, mediated by elevated c-Fos levels in the dentate gyrus of rats 45 .Alongside c-Fos, the expression of EGR-1/zif268 and ARC also increases, further promoting neurogenesis activity in the hippocampal granule cell layer 46 .Voluntary running exercise induction also boosts c-Fos expression through Akt phosphorylation in the female rat hippocampus, enhancing adult hippocampal neurogenesis for cell survival, even in rats with Huntington's disease 19 .The expression of c-Fos also increases after treadmill exercise induction, influencing learning and spatial memory 18 .Cold water swimming is associated with increased activation of hippocampal interneurons and a higher number of new neurons in the dentate gyrus 16 .
Grinspun et al. research in 2019 47 explained that running wheel exercise improves memory, as evaluated by increased c-Fos immunoreactivity in the tuberomammillary nucleus (TMN).Similarly, treadmill exercise is shown to benefit several neurodegenerative diseases, such as Alzheimer's, by increasing c-Fos expression in the hippocampus in mice after streptozotocin (STZ) injection, reducing long-term memory deficits 48 .In rats with diabetes induced by STZ injection, subsequent treadmill exercise can enhance neuroplasticity and spatial memory by upregulating the expression of the c-Fos nerve gene in the hippocampus 49 .Increased c-Fos expression resulting from physical exercise and therapy aims to boost neural activity, learning, and memory, even in children born to alcoholic mothers during pregnancy, thus countering the effects of alcohol poisoning on postpartum mothers and their babies 50 .Swimming exercise has been found to mitigate isoflurane-induced memory impairment by increasing the expression of c-Fos, a CREB-binding protein (CBP), which elevates hippocampal histone acetylation and activates more neuron cells during memory formation, ultimately improving memory impairment 20 .
Exercise intensity plays a significant role in c-Fos expression.High-intensity treadmill exercise increases c-Fos expression and brain-derived neurotrophic factor (BDNF) levels in the hippocampus, thereby enhancing memory through heightened neuroplasticity in both the hippocampus and prefrontal cortex 21 .Foley et al. study also demonstrated that the induction of c-Fos mRNA expression at high intensities significantly increases c-Fos mRNA activation in the hippocampus, affecting the pattern of potential brain activation compared to low-intensity exercise 51 .Low-intensity acute and long-term treadmill exercise, performed at a speed of 10 m/min over four weeks, induces c-Fos expression by increasing the density of c-Fos+ cells in various brain regions, including the striatum, primary somatosensory and secondary motor cortex, hippocampal subregion, nucleus hypothalamus, and lateral periaqueductal grey.This positive correlation with BDNF expression suggests that long-term exercise has a broader regional and temporal impact on the brain than acute exercise 22 .Moderate-intensity treadmill exercise has proven more efficient in activating hippocampal nerves than rotarod exercise 14 .
In addition to exercise intensity, the type of exercise also significantly influences c-Fos expression in the hippocampus, especially in treadmill and rotarod exercises.Immunohistochemical examination of c-Fos as a nerve activation marker revealed that treadmill exercise at 15 m/min (T15) significantly increases c-Fos expression in all hippocampus subfields, including CA3, CA1, and the dentate gyrus.In contrast, 30 rpm rotarod exercise (R30) does not result in increased c-Fos expression 14 .
The duration of exercise in voluntary wheel running over 25 days is found to increase the survival of new neurons and affect c-Fos expression.Longer exercise duration has a greater impact on the rate of neurogenesis, as it leads to increased differentiation of hippocampal progenitor cells in adult rat models 52 .The expression of c-Fos induced by physical exercise is influenced by exercise intensity, type, and duration, with higher intensity and longer durations resulting in increased c-Fos expression in the hippocampus 53,54 .
Increased c-Fos expression in the rat hippocampus due to treadmill exercise was observed across all age groups.The cornu ammonis (CA) region showed the highest increase in c-Fos expression with treadmill exercise in 4-week-old rats, while in the dentate gyrus, the highest increase was observed in rats aged 62 weeks.This suggests that age is a crucial factor in regulating c-Fos expression in the hippocampus, as it appears to be age-dependent when induced by exercise 55 .
Expression ARC during exercise ARC is one of the most characteristic molecules that play a key role in memory formation.This gene encodes proteins involved in synaptic functions related to serotonin, glutamate, and dopamine, distinguishing it from c-Fos and EGR-1 [56][57][58][59] .ARC expression is regulated by EGR-1 60 and its mRNA is transported to dendrites 61,62 , making it a marker of neural activity 56,63 .Under certain conditions, mRNA ARCs can form in postsynaptic dendrites independently of presynaptic axons 64,65 .ARC acts on both new and established synapses, playing a crucial role in mechanisms related to synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD) 3,66 .ARC encodes an F-actin-associated (ARC) 10,67 growth factor that contributes to dendritic reconfiguration 62,68 .Therefore, both the ARC and EGR-1 genes, albeit to varying degrees, influence changes in neural tissue and play more specific roles than c-Fos 69 .
Chronic aerobic exercise improves working memory and can be evaluated as a neuroprotective measure due to increased ARC and BDNF expression in the hippocampus.This effect was observed in rats subjected to a Western diet combined with a plant-based Daniel Fast (DF) intervention 11 .The expression of ARC induced by physical exercise is influenced by age, with physical exercise during adolescence increasing the expression of genes related to synaptic plasticity and cognitive function in the hippocampus, including synaptophysin, BDNF, PSD-95, CREB, ARC, TLX, and DCX.However, physical exercise in adulthood does not affect the expression of these genes, suggesting that exercise during adolescence is more effective in enhancing cognitive function 12 .This finding was supported by Sun et al. research in 2021 13 whose research explained that treadmill exercise can improve neuroplasticity in male rats exposed to (2-ethylhexyl)-phthalate DEHP before birth, leading to memory and spatial learning deficits during late adolescence.Exercise during childhood and adolescence can restore gene expression and improve learning and memory deficits 16 .In old age, wheel running exercise increases the expression of genes such as BDNF, FNDC5, PGC-1α, mTOR, ARC, c-Fos, ERK, SIRT, and FOXO.This can help counter cognitive dysfunction associated with aging, which is characterized by decreased expression of genes and proteins like PGC-1α, FNDC5, and BDNF in the hippocampus 23 .
Expression EGR-1 during exercise EGR-1, known by various names such as zif268, NG-FI-A, Krox 24, or ZENK 70,71 , is a transcription factor whose expression is induced by various factors, including injury, stress, cell differentiation, and extracellular signals such as growth factors, neurotransmitters, and peptides [72][73][74] .EGR-1 expression exhibits a distinct pattern in the brain compared to c-Fos [68][69][70][71][72][73][74][75] .It plays a crucial role in mediating the expression of multiple genes involved in neural processes, ranging from growth control to changes in synaptic plasticity [76][77][78] .EGR-1 is relatively highly expressed during neural activity 72,79 , particularly in the hippocampus 3,80 and the dentate gyrus 7 .Its role in learning and memory is attributed to its modulation of synaptic plasticity, including the remodeling of dendrites, and synapses, and the formation of new synaptic connections 69 .
This study supports the role of EGR-1 in learning and memory formation, affecting neural and cognitive functions.EGR-1 functions as a transcription factor that regulates numerous identified target genes.In contrast to c-Fos, which primarily targets genes related to vesicular transport and neurotransmitter release, EGR-1 targets genes that often depend on clathrin or actin processes 81,82 .
Physical exercise also influences EGR-1 expression, with different durations of exercise affecting EGR-1 expression in the hippocampus 83 .However, research on the effects of exercise on EGR-1 expression is still limited, and conclusive findings on its impact in the hippocampus cannot be drawn at this time.

DISCUSSION
The review analysis above demonstrates that environment and exercise influence the expression of EGR-1, ARC, and c-Fos genes in memory formation and storage.C-Fos gene expression serves as an indicator of neuronal activity.When neurons are active, the c-Fos gene becomes activated, leading to increased production of c-Fos protein.This protein plays a crucial role in neuroplasticity, involving structural and functional changes in the brain.Its role in the hippocampus has been investigated in mice, where c-Fos is involved in neurogenesis and the significance of the AP-1 transcription factor in c-Fos development 84 .The activation of the c-Fos gene and protein occurs rapidly after stimulation and depends on the type and timing of the stimulus.C-Fos requires the participation of other genes, such as c-Jun, for its expression during heterodimer formation.C-Fos dimerizes with the c-Jun protein to form the AP-1 factor, which promotes the transcription of various genes.While the production and removal of c-Fos are part of cellular homeostasis, overexpression can lead to increased cell proliferation 85 .
Similarly, ARC expression is influenced by physical exercise in the hippocampus.When we engage in physical exercise, synaptic activity in the hippocampus increases, facilitating communication between neurons.This heightened synaptic activity triggers the expression of the ARC gene.The primary role of ARC protein is in the process of neuroplasticity.When the ARC gene is activated, ARC proteins are produced and move to the synapses, where neurons communicate.ARC protein acts as a bridge between synaptic signaling and structural changes in neurons, regulating synaptic changes and promoting the formation and maintenance of new synaptic pathways 86 .
In the context of the hippocampus, exercise-induced c-Fos and ARC gene expression may facilitate synaptic changes that support learning and memory.The ARC protein also plays a role in memory consolidation, converting short-term memory into long-term memory.Additionally, both c-Fos and ARC proteins are involved in the storage and reactivation of existing memories 66,87 .Therefore, increased ARC gene expression due to physical exercise can influence neuroplasticity in the hippocampus, including memory formation and maintenance.Research has also demonstrated that a deficiency in the ARC gene can impair cognitive performance and memory, underscoring the critical role of this gene in brain function 88 .
ARC expression is regulated by EGR-1 60 .EGR-1 is a transcription factor involved in gene regulation in cells, and its activation can affect the expression of genes involved in synaptic plasticity, learning, and memory in the hippocampus.EGR-1 gene expression plays a pivotal role in the brain's response to physical exercise in the hippocampus.EGR-1 is a transcription factor activated by external stimuli, including physical exercise.Physical exercise is shown to increase EGR-1 gene expression in the hippocampus.During physical exercise, there is an increase in synaptic activity and the release of neurotransmitters in the hippocampus, resulting in an external stimulus that triggers the activation of the EGR-1 gene 46 .
The primary function of the EGR-1 protein is to regulate the expression of other genes in response to stimuli.The EGR-1 protein acts as a transcription factor, binding to the promoter of the target gene and regulating the production of that protein.In the context of the hippocampus, exercise-induced increased expression of the EGR-1 gene influences other genes involved in synaptic plasticity, neurogenesis, and cognitive function 82 .
Although numerous studies demonstrated a relationship between physical exercise, c-Fos, ARC, EGR-1 gene expression, and hippocampal function, this area remains a subject of ongoing research.In particular, research into the effect of exercise on EGR-1 gene and protein expression is still limited.Therefore, further research is needed to understand the impact of physical exercise on EGR-1 gene expression, hippocampal function, and other factors such as the type of exercise, intensity, and duration, which can also influence the brain's response.
In conclusion, the expression of IEG genes, including c-Fos, EGR-1, and ARC, in hippocampal neurons increases after being induced by exercise.This beneficial process in synaptic plasticity is associated with learning, memory, and neurogenesis.

Figure 1 .
Figure 1.Flowchart of the process of selecting research studies that have been identified and elected based on criteria, research processes, and databases.

Figure 2 .
Figure 2. C-Fos, ARC, EGR-1 expression during exercise: physical exercise triggers physiological changes in the body by myokine expression in the muscles.Activation of membrane receptors in the brain: the neurotransmitters released bind to specific receptors in the brain, including in the hippocampus.This activation causes a cascade of intracellular events.Opening signaling pathways: the binding of neurotransmitters to their receptors initiates signaling pathways within hippocampal neurons.The cyclic adenosine monophosphate (cAMP) pathway is involved, CaMkII/IV.The cAMP pathway: activation of the cAMP pathway triggers a cascade of intracellular reactions, activating protein kinase A (PKA).PKA is an enzyme that phosphorylates and activates other proteins in gene expression.Once activated, PKA translocates to the nucleus of hippocampal neurons, where it phosphorylates and activates transcription factors such as CREB.Gene expression: CREB binds to a specific DNA sequence called the cAMP response element (CRE) in the promoter region of the target gene.This binding initiates a transcription of these genes directly to the start gene (c-Fos, ARC, EGR-1).Exercise stimulates neurotransmitter release, activates signaling pathways, phosphorylates transcription factors, and leads to c-Fos, ARC, and Egr-1 expression in the hippocampus.The activation of this gene indicates increased neural activity, synaptic plasticity, and cognitive benefits associated with exercise.

Table 1 .
Bibliographic sources identified, type of exercise, technique used, sample, intervention, and outcomes.